Certain advances have recently been made in the field of electrostatic printers which substantially simplify the application of ink or toner particles to a sheet of copy paper or the like. Generally, such electrostatic printers employ a corona source and a spaced electrode for generating a substantially uniform ion stream, and a support for positioning a print receiving medium in the path of the ion stream. A multi-layered apertured two-dimensional screen or line grid modulator is interposed in the ion stream between the source and print receiving medium for modulating the cross-sectional flow density of ions in the stream in accordance with a pattern to be reproduced. A cloud of substantially uncharged toner or marking particles is formed adjacent the print receiving medium whereby the modulated ion stream selectively impinges upon and charges toner particles in the cloud. The selectively charged toner particles adjacent the print receiving medium are thereafter accelerated and deposited on the medium in accordance with the pattern to be reproduced.
Such a system of electrostatic printing is set forth in greater detail in the U.S. patent application Ser. No. 101,681, filed Dec. 28, 1970 for "ELECTROSTATIC PRINTING SYSTEM AND METHOD USING IONS AND TONER PARTICLES", now U.S. Pat. No. 3,779,166 which is assigned to the assignee of the present application. According to the disclosure in the referenced patent application, modulation of the ion flow is accomplished by using a multi-layered apertured element spaced between the ion source and the accelerating electrode. The element has at least a conductive layer and an insulative layer capable of supporting charge potentials of differing magnitude on different layers of the element for establishing electrostatic lines of force within the apertures of the element for controlling passage of ions in accordance with a pattern to be reproduced. Suitable multilayer apertured elements are shown in Pressman U.S. Pat. No. 3,689,935. The corona or ion source and the spaced electrode generate a substantially uniform stream of ions which has a line or linear cross-sectional configuration. The multilayered apertured element is capable of supporting charge potentials of differing magnitude on different layers of the element for establishing electrostatic lines of force within the apertures of the element for controlling passage of ions. The print receiving medium is supported and positioned between the modulating element and the accelerating electrode in the path of the linear ion stream and the print receiving medium is transported across the line cross-section of the ion stream at a given speed.
The cloud of substantially uncharged toner marking particles is between the modulating screen and the print receiving medium and the modulated linear cross-section ion stream selectively impinges upon and charges toner particles in the cloud. Charged particles are accelerated and deposited on the print receiving medium in accordance with the pattern to be reproduced. Normally, a velocity component is imparted to the toner cloud substantially equal to and in the direction of motion of the print receiving medium.
The sharpness or resolution of the spot patterns formed on the medium, say of alphanumeric characters on a sheet of print-out paper, is primarily a function of the voltage difference and of the strength of the electric field between the modulator and the electrode. The stronger the field, the sharper the print-out because toner particles impinged on by ions are more rapidly deposited on the paper and stray less from their straight line path. It appears therefore that the obvious way to improve print-out resolution is to increase the strength of the electric field. To a limited extent, this is possible. However, excessive increases in electrode voltage cause the electrode to form a corona and produce ions with an opposite charge to the charge of the ions emanating from the modulator. The corona and ions generated at the electrode are called secondary corona and secondary ions. The secondary ions travel towards the aperture through the toner cloud. There, they impinge on particles and cause particle movements to the modulator where the particles are deposited. The toner deposit builds up and may clog the apertures of the modulator, which would render the modulator and print-out mechanism inoperative. Thus, increasing the electric field strength to enhance print-out resolution does not ordinarily yield the desired result.